Korupt is right.
Let me simplify this:
If you put spy protection of 50 ticks on your colony/outpost when someone already planted a agent on your colony/outpost which has let's suppose 100 ticks of infiltration he will always be able to spy attack you.
Finally 100 ticks of infiltration > 50 ticks of spy protection = your colony is vulnerable to spy attacks.
No blue George.